You may have a couple different problems. the thermostat may be stuck open hterefore not allowing your engine to heat up properly. the coolant will just circulate through the engine and rad without heating up to proper temp. the other problem you may have is that you might have a plugged heater core thus not allowing the coolant to circulate through the core and give you heat.
I would recomend checking the thermostat and see if it is operating properly. an easy way to check it is by opening your rad when the engine is cold. start your truck and watch the coolant in the rad. If the thermostat is stuck open you should see the coolant circulating through the rad. This is not always the best way to check but it is an easy way to see. the other way which involves a little more work is to take the thermostat housing off and physically inspecting the thermostat.
hope this helps you out

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First thing I would do is flush the heater core with a 4 to 1 mixture of water and muratic acid. Put it in there and blow it out both ways 3 or 4 times. Let the acid mix set for about 10 minutes before blowing it out with air. This is a cheap fix, if it works. $7 and your time. Got to say though, it only works about 30% of the time. The other 70% is to remove the heater core. Dodges/Chrysler su(k. To get to the heater core, you have to drop the steering column, and remove the entire dash. Then remove the heater/A/C plenum to remove the heater core. Buy a manual to do this. Way to many hidden screws and bolts. This job costs about $800 at a garage. It takes about 8 hours to do, if you know what your doing.

Chances are it is indeed the heater core as even a spent heater unit will blow hot air off the engine when the ambient temps are hot . When the temps drop, the heat from the engine block is not enough to heat the cab properly. Have you tried the old cardboard in front of the radiator trick? Speaking of semi's,ever notice they cover their radiators in the winter? That's because it helps get their block temps up a little higher and assists with heating as well. It's not much help but do the buyer a favour and let them know beforehand. Have a good one. :)

i hate no heat. ford taurus 2001 are a problem sometimes with heat issues.on the firewall there are heater hoses that get clogged,pinched,and sometimes it's just a bad heater core. you said everything you can think of has been done?[replaced heater core? flushed the radiator? are the cooling fans coming on? is radiator full of coolant? have you took the cap off to be sure?like i said,i hate no heat. i'm still thinking.thermostat new?the blower works but only blows cold air?ok if thermostats good,radiator is full,blower works,radiators full,cooling fans coming on, no engine overheating but temperature is fine,let's do this for a short winter time solution only!in the front of your car with the hood up.look at the radiator,ok,in front of the radiator,place a peice of cardboard in front of radiator on the bumper side. think i'm crazy right? try this. cut just a small peice of cardboard about half the size of the radiator and insert it in FRONT BUMPER SIDE of the radiator, this will give you emegency heat,it will have to come out when the temperature outside warms up because it will make your car overheat.for sure.but in cold weather,it's a sure thing to bring the heat up inside the car. you are restricting the air that flows through your radiator while the cars moving down the road,do not let car idle as it will heat up,as long as you're moving you will be safe,keep an eye on your temp. guage as you may have to make an adjustment to the size of the cardboard. yep i was a truck driver for many years and know so well how cold the road is. i kept my cardboard handy in the winter and never got cold. you do have an existing problem with your heat,and unless you deal with it soon,you'll need those cardboard boxes and duct tape for a minute.if you don't beleive me,try it.you'll be toasty in no time,just watch that temp guage and you'll be fine. thank you for choosing fixya.com and by the way,let me know how it works for you,i love to hear that i've assisted someone,somewhere.

it sounds like a vaccume leak check under hood for vaccume line that goes through the firewall trace this line and look for leaks next pull controll unit out of dash most times this unit is worn an will leak causing lak of controll of the heat an airconditioning

Sounds as if the engine is not building up temp due to cooling system working. If engine temp gage is reading low try covering 1/3 of radiator with a piece of cardboard "Bumper side" to reduce the cooling effect.. just watch temp and out side temps if warming up outside or heavy driving remove the blocking to allow full cooling

You probably just need to "burp" your cooling system. Run the car (from cold) with the radiator cap open and the heater on full blast. As the engine heats up and the water level goes down, fill it up until it stays full and starts to overflow.

as a test, restrict a good portion of front of radiator with cardboard. is the engine running at 195 degrees? if engine temp is good, verify that both heater hoses going to heater core are hot. maybe heater control valve.